The present invention relates to an electronic switching device, or component, for the switching networks of telephone central offices, the switching device including a switching transistor disposed in the series branch in the signal conduction path and controllable semiconductor devices in a shunt branch which is disposed transversely to the signal conduction path. In the conductive state of the switching transistor, the device has a low series resistance and a high shunt impedance and in the blocking state, the device has a high series resistance and a low shunt impedance.
German Auslegeschrift [Published Patent Application] No. 1,293,214 discloses an electronic switching device with bistable behavior, for the switching of lines in telegraph and particularly telephone central offices of the above-mentioned type, in which the shunt resistance is a voltage-dependent resistor whose impedance can be changed between different values by the control voltage which is applied to the transverse branch and which is dependent on the switching state of the switching device. In this publication it is proposed to connect a variable capacitance diode in parallel with either a resistor, a bipolar transistor or a pn field effect transistor for the controllable transverse branch.
This switching device is limited to using semiconductor devices with bistable behavior in the series branch. In order to keep this switching device in a conductive state after it has been switched on, it is necessary for the direct current flowing therethrough to always be greater than the holding current. Furthermore, interfering pulses in the voice line may cause the switching device to be inadvertently automatically switched off or on.
The magazine Bulletin Technique PTT 2 (1973), describes on pages 79-83 a fully integrated space-multiple switching device which is based on the conventional transistor switching device. It includes a bipolar transistor in the series branch and a fixed ohmic resistor in the transverse branch which serves to switch on and off the series branch transistor by means of a control circuit. In order to produce the switching device in a monolithically integrated form, it is modified so that a npn transistor is used in the series branch as the switching element and instead of the fixed resistor a current source in the form of a pnp lateral transistor and an npn transistor are used as the control in the transverse branch. Both transistors have their collector terminals connected to the base terminal of the series branch transistor. The npn transistor in the transverse branch is controlled by a holding flip-flop. If the npn transistor is switched on, i.e. rendered conductive, the series branch transistor is blocked, if the non transistor is switched off, i.e. placed in its blocking state, the series branch transistor is rendered conductive by the pnp lateral transistor.
The above-described circuit, which is the monolithic equivalent of the conventional circuit, has the drawback that the pnp lateral transistor is always switched on, regardless of whether the series branch transistor is blocking or conductive. Thus there continuously exists a relatively high direct current energy loss in the switching device. The result is that the packing density of the semiconductor switching devices in a switching matrix module produced from these semiconductor switching devices is limited, since with the small space occupied by the switching matrix module the energy consumption is a significant parameter because: (1) the operating dependability of the semiconductor switching devices is a function of the crystal temperature of the semiconductors; and (2) the energy loss per housing is limited to about 200 mW and thus determines the degree of integration per housing.
The above-described matrix module thus contains only 2 .times. 2 symmetrical semiconductor switching devices including the control circuit on a single semiconductor chip.
A further drawback of the described switching device is that the series branch transistor requires a directional DC collector operating current which limits the freedom of choice for the speech path in the switching network to connections from one side to the other and requires complicated junctor circuits with current sinks and capacitively or inductively coupled switching. A relatively high energy consumption in the junctor circuits is the result. Also, the direct collector current (I.sub.C .apprxeq.10mA) which flows through one series branch transistor into the junctor circuit is extremely high.